1. Field of the Invention
The present invention relates generally to a channel communication method in a mobile communication system, and in particular, to a communication method for readily setting a secondary scrambling code in a mobile communication system which expands a channel capacity using a plurality of scrambling codes.
2. Description of the Related Art
In general, a CDMA (Code Division Multiple Access) communication system uses scrambling codes for identification of base stations. The scrambling codes are also used for an increase in the channel capacity of the base stations as well as identification of the base stations.
A UMTS (Universal Mobile Telecommunication System) communication system, which is a European W-CDMA communication system, uses a plurality of scrambling codes for identification of the base station and an increase in the channel capacity of the base stations. In the UMTS system, when a base station has used up all the orthogonal codes assigned to one scrambling code and thus has no more available orthogonal code, the base station uses another scrambling code to expand the channel capacity. That is, the base station sets a new scrambling code and then assigns orthogonal codes for the newly set scrambling code. To generate the scrambling codes, a Gold sequence of length 218-1 is typically used. In the Gold sequence of length 218-1, 218-1 different Gold codes constitute one group. For the scrambling codes, the Gold code of length 218-1 is repeatedly selected by 38400 bits from the first bit.
In general, the scrambling code used for identification of the base stations is referred to as a “primary scrambling code”. The primary scrambling code and orthogonal codes using the primary scrambling code are then assigned. If the orthogonal code is insufficient to assign for newly adding channels using the primary scrambling code, another scrambling code is set and then orthogonal codes using the set scrambling code are assigned. The scrambling code used at that case is referred to as a “secondary scrambling code”. That is, the number of the orthogonal codes which can be assigned using the corresponding scrambling code is determined by the data rate of presently communicating channels. Therefore, it is possible to expand the channel capacity by providing a plurality of the scrambling codes and setting an unused scrambling code when the channel capacity is insufficient.
The primary scrambling code is used for identification of the base stations and for scrambling the signal spread with the assigned orthogonal codes. It will be assumed herein that the number of the primary scrambling codes is 512. Therefore, adjacent base stations use different primary scrambling codes out of the 512 primary scrambling codes.
In general, the mobile stations identify the base stations by analyzing the primary scrambling codes. Therefore, the base station transmits the common control channels to the mobile stations using a unique primary scrambling code, and transmits the downlink channels using either the primary scrambling code or the secondary scrambling code according to the present channel capacity.
In general, the base station transmits the common control channels to the mobile stations using a unique primary scrambling code, and transmits the downlink channels using either the primary scrambling code or the secondary scrambling code according to the present channel capacity. Therefore, the mobile stations identify the base stations by analyzing the primary scrambling codes.
The secondary scrambling codes used to increase the channel capacity of the base stations correspond to the primary scrambling codes used in the base station, and the maximum number of the secondary scrambling codes is 512. The base station selects the secondary scrambling codes.
Reference will now be made to UMTS downlink transmission for which several scrambling codes are used.
FIG. 1 illustrates a downlink channel transmitter of a UMTS base station. Referring to FIG. 1, a dedicated physical control channel DPCCH and N dedicated physical data channels DPDCH1 to DPDCHN are applied to demultiplexers 100 to 104, respectively, after channel coding and interleaving. The demultiplexers 100–104 demultiplex DPCCH and DPDCH1–DPDCHN into I and Q signal components, respectively. The I and Q signal components output from the demultiplexer 100 are applied to multipliers 110 and 111, which multiply the received I and Q signal components by a first orthogonal code for channel separating of the I and Q signals. A scrambler 120 scrambles the multiplied signals. The demultiplexers 102–104 have the same operation as the demultiplexer 100, multipliers 114, 115, 118 and 119 have the same operation as the multipliers 110 and 111, and scramblers 124 and 128 have the same operation as the scrambler 120.
A scrambling code generator 150 generates scrambling codes and provides the generated scrambling codes to the scramblers 120, 124 and 128. The scrambling codes generated by the scrambling code generator 150 include the primary scrambling codes, and the secondary scrambling codes for increasing the channel capacity of the base stations. The scrambling code generator 150 provides the primary scrambling codes to the scramblers that use the primary scrambling codes, and the secondary scrambling codes to the scramblers that use the secondary scrambling codes.
The scramblers 120, 124 and 128 each complex-multiply the multiplied input signals by the corresponding scrambling codes, and provides the resulting real part components to a summer 130 and the resulting imaginary components to a summer 135. The summer 130 sums the real part components of the scrambled signals and the summer 135 sums the imaginary part components of the scrambled signals.
FIG. 2 illustrates a detailed structure of the scrambling code generator 150 of FIG. 1, which simultaneously generates several scrambling codes.
Referring to FIG. 2, the common control channels normally use the primary scrambling codes. However, when there is an insufficient number of the orthogonal codes, the downlink dedicated channels should use the secondary scrambling codes. Therefore, it is necessary for the base station to be able to generate a plurality of scrambling codes. In FIG. 2, control information #1 to control information #N of scrambling codes for several channels are applied to N Gold sequence generators 211–21N, respectively. The Gold sequence generators 211–21N generate Gold codes corresponding to the received control information #1 to control information #N, and output the I-channel components unchanged and provide the Q-channel components to corresponding delays 221–22N. The delays 221–22N delay the received Q-channel components for a specific chip period.
FIG. 3 illustrates a downlink channel receiver of a UMTS mobile station. The receiver be able to descramble the received down link common control channel signals that were scrambled with the primary scrambling code in the base station. And should also be able to descramble other received downlink channels, which were scrambled with the primary scrambling codes or the secondary scrambling codes in the base station. Therefore, the receiver should be able to generate a plurality of scrambling codes to descramble the received downlink channels.
In FIG. 3, the I and Q components of the signals received at the mobile station are applied to descramblers 310 and 315, respectively. A scrambling code generator 300 simultaneously generates primary scrambling codes and secondary scrambling codes for respective channels, and provides the generated scrambling codes to the descramblers 310 and 315. The descramblers 310 and 315 multiply the received signals I+jQ by conjugate values of the scrambling codes provided from the scrambling code generator 300 to despread (descramble) the received signals, and provide the descrambled I and Q components to multipliers 320–326. The signals output from the descramblers 310 and 315 are applied to the multipliers 320–326 where the signals are multiplied by orthogonal codes for the corresponding channels, for despreading. Thereafter, the despread signals are multiplexed by multiplexers 330 and 335.
FIG. 4 illustrates a detailed structure of the scrambling code generator 300 of FIG. 3, which simultaneously generates several scrambling codes. In the base station for the mobile communication system, which uses the scrambling codes, the common control channels are normally scrambled with the primary scrambling codes and other channels are scrambled with either the primary scrambling codes or the secondary scrambling codes according to the system capacity. Therefore, the mobile station should be able to generate the secondary scrambling codes as well as the primary scrambling codes. In addition, since the signal scrambled with primary scrambling code and the signal scrambled with secondary scrambling code can be simultaneously received, it is necessary for the mobile station to be able to simultaneously generate the primary scrambling codes and the secondary scrambling codes.
Referring to FIG. 4, upon receipt of control information #1 and control information #2 of scrambling codes for the respective channels, Gold sequence generators 411 and 412 generate Gold codes corresponding to the control information #1 and #2. At this point, the I components of the generated Gold codes are output unchanged, and the Q components are delayed by the corresponding delays 421 and 422 for a specific chip period.
FIG. 5 illustrates a detailed structure of the Gold sequence generators of FIGS. 2 and 4. In general, a Gold sequence is generated by XORing two different m-sequences. In FIG. 5, an m-sequence generator polynomial of an upper shift register 500 is f(x)=x18+x7+1, and a generator polynomial of a lower shift register 510 is f(x)=x18+x10+x7+x5+1.
The number of Gold codes generated by the Gold sequence generator of FIG. 5 is 512*512=262,144. The Gold codes generated by the Gold sequence generator are divided into the primary scrambling codes and the secondary scrambling codes. Of the 261,144 Gold codes, 512 are the primary scrambling codes, and 511 Gold codes are associated with each primary scrambling code, constituting a set of the secondary scrambling codes.
The 512 primary scrambling codes are generated by setting 512 upper shift register initial values and XORing the output of upper shifter register 500 and the lower shift register 510. Here, the upper shift register 500 has a binary value of a decimal number of 0 to 511 as an initial value, and the lower shift register 510 normally has a value of ‘1’ at every shift register as an initial value. The secondary scrambling codes are generated by providing i+512*k as an initial value of the upper register 500, where ‘i’ denotes a code number of the primary scrambling code and ‘k’ denotes a value of 1 to 511. Therefore, each primary scrambling code is associated with 511 secondary scrambling codes. Each base station uses one primary scrambling code, and uses one or more secondary scrambling codes as occasion demands.
The primary scrambling codes are necessarily used when scrambling a primary common control channel (P_CCPCH). Other downlink physical channels are scrambled with either the primary scrambling signal or a secondary scrambling code selected from the secondary scrambling code set, before transmission.
As described with reference to FIGS. 1 to 5, there can be used several scrambling codes at the request of the base station. Therefore, the base station should include a scrambling code generator, which can simultaneously generate several scrambling codes, and the mobile station should also have a scrambling code generator, which can generate several scrambling codes, in order to correctly receive the signals transmitted from the base station.
Referring again to FIG. 5, the Gold sequence generator cannot simultaneously generate several scrambling codes, and generates only one scrambling code at a time. Thus, to generate several scrambling codes, it is necessary to provide a number of the Gold sequence generators equal to the number of the scrambling codes.
In addition, the number of the scrambling codes generated by the Gold sequence generator of FIG. 5 is 262,144 in total. Each base station can perform communication even with one primary scrambling code and 511 secondary scrambling codes associated with the primary scrambling code. It is not difficult for the base station to store 262,144 scrambling codes, considering its large memory capacity. However, the mobile station, which performs communication while traveling between base stations, cannot know which primary scrambling code and secondary scrambling code are used by the base stations, the mobile station should store all the 262,144 scrambling codes. A storage area for storing the 262,144 scrambling codes will occupy a considerable storage area of the mobile station, considering the small memory capacity of the mobile station.
Further, in the case where the scrambling codes are generated using the Gold codes of FIG. 5, when there are an insufficient orthogonal codes for the primary scrambling codes, the base station should inform the mobile station of information about a secondary scrambling code which will be using, while transmitting the channel signals which were scrambled with the secondary scrambling codes. However, since the base station should transmit one of the numbers of 512 to 262,144 indicating the secondary scrambling code, the base station should transmit 18-bit information about the secondary scrambling codes.